1. Field of the Invention
The present invention relates to a direct injection engine for directly injecting fuel into a combustion chamber and a controller for the engine. More particularly, the present invention relates to a direct injection engine and a controller for the engine, in which the shape of a piston, the shape of fuel spray, etc. are improved to reduce an emission amount of HC at startup of the engine and the ignition timing is largely retarded from that in the ordinary operation to realize super-retard combustion under a high back pressure.
2. Description of the Related Art
In a direct injection engine, many proposals have hitherto been made of methods for improving characteristics (such as the shape and penetration) of fuel spray injected from a fuel injection valve, the shape (of a crown surface) of a piston, flows of air in a combustion chamber, etc., to thereby stratify a gas mixture and improve fuel economy. Recently, those proposals have been progressed from a method of forming a cavity (recess) with a special shape in the piston and stratifying a gas mixture with the presence of the cavity, to a method of utilizing flows of air, etc. to reduce an amount of fuel deposited on a wall surface of the combustion chamber and cut an emission amount of HC, and then to a method of spraying fuel so as to directly reach an ignition plug for an improvement of combustion stability.
As one of those methods, JP-A-2000-120440, for example, proposes a direct injection engine in which two cavities are formed in the crown surface of a piston to have an ω-shaped section. Then, fuel is sprayed to collide against one of the two cavities, while a forward tumble is formed in the other cavity. The fuel spray and the tumble are forced to collide against with each other, thereby promoting mixing of the fuel and air while causing a combustible gas mixture to reach an ignition plug.
Also, JP-A-2002-295261 proposes a direct injection engine in which a step giving a level difference is formed in a bottom wall of a cavity in its portion near the exhaust side, and fuel spray is blown up by the presence of the step to collide against a tumble (vertical vortex) formed in a combustion chamber for stratification around an ignition plug.
Those direct injection engines are advantageous in that, during stratified charge combustion, atomization of the fuel and mixing of the fuel with air are promoted with interaction between the fuel spray and the airflow so that the combustible gas mixture forced to exist in the vicinity of the ignition plug.